As indicated in the above document in particular, the “Smart Cut®” process consists in implanting a starting substrate (such as a silicon plate or wafer, in practice having an oxidized surface) with hydrogen ions (see FIG. 1) to create a weakened zone 3 (see FIG. 2). This first plate is then transferred onto a second substrate 4 (for example a second silicon plate or wafer) using an appropriate bonding technique (see FIG. 3), for example molecular bonding. The resulting bonded assembly is then placed in a furnace to undergo annealing during which defects coalesce (see FIG. 4) and at the end of which a layer, for example a thin layer, is transferred (see FIG. 5), i.e. the layer 1A of the starting substrate situated between the surface and the weakened zone is detached from the remainder of the starting substrate, remaining bonded to the second substrate.
This heat-activated process leads to maturing of the defects generated by the weakening and propagation of microcracks (caused by those defects), up to the point of complete transfer of the microtechnological layer, by separation or fracture along the weakened zone.
The thickness of the layer obtained in this way, when it is a thin layer (the expression thin film is sometimes also used), is typically a few hundred nanometers, while the thickness of the substrate is approximately 700 μm (see above); it is clear that the starting substrate can then be subjected to repetition of the process to form a number of thin layers in succession.
In a first embodiment of the method, the transfer takes place during the heat treatment (possibly combined with the application of complementary forces, such as mechanical stresses), while in a second embodiment the heat treatment is followed by a complementary treatment (for example involving the application of forces, such as mechanical stresses) during which the transfer takes place.
Depending on whether the heat treatment is intended to bring about the transfer separation or must be interrupted before such separation, the benefit that there would be in knowing how to measure the duration of the annealing that leads to such separation or how to define the moment at which separation occurs is clear. If separation is to occur during the heat treatment, it is of no benefit to extend the heat treatment beyond separation, for obvious reasons of saving energy; on the other hand, if separation is to occur only after the heat treatment, it can be of benefit, as a safety measure, to be able to detect that unintentional separation has occurred and to interrupt the heat treatment before other items from the same batch also suffer unintentional separation; microtechnological layers are often produced in batches, and it is of benefit to be able to interrupt the heat treatment at the latest at the moment when one of the items being treated is subject to separation, to protect the other items and the equipment in which the heat treatment is effected, to prevent excessive contamination caused by the items breaking.
Consequently, the benefit of being able to detect such fracture or separation automatically or at least in a manner that lends itself to automation is clear. Knowing how to measure the duration of the heat treatment before the transfer separation or how to detect such separation is of particular benefit if the separation technology is applied to substrates consisting of new materials, i.e. if there is as yet insufficient data to be able to predict an optimum duration for the heat treatment.
Until now, this detection has been done by ear, because fracture produces a sound that is in theory audible, which requires close attention if it is to be discerned with certainty, given the generally noisy environment, especially if there is a batch of items the respective fractures in which are to be detected. There are many sources of unwanted noise: ventilation, which can be intense (in the room containing the furnace), opening or closing a door, movement of persons in the room, etc. The transfer heat treatment can be carried out at a temperature of several hundred ° C. and can have a duration from a few minutes to several hours.
At present, detecting this sound is tiresome and often somewhat uncertain.